This Small Business Innovation Research (SBIR) Phase I project aims to develop low-cost solution-processed transparent conducting electrodes for tandem solar cell applications. Tandem solar cells can be comprised of two vertically stacked thin film solar cells that are each designed to absorb different regions of the solar spectrum, allowing an efficiency that is higher than a single junction device. Tandem solar cells are a promising technology that is capable of achieving more than 25% power conversion efficiency. However, a highly-efficient top cell that is more than 75% transparent in the near-infrared spectral region is required. Currently, transparent electrodes are primarily made of metal oxides, which strongly absorb in the near-infrared portion of the solar spectrum, significantly reducing the potential tandem power conversion efficiency. In this project, PLANT PV will develop new materials that are highly transparent in the near infrared and can withstand high processing temperatures to lower the cost and improve the overall power conversion efficiency of tandem solar devices.
The broader/commercial impact of this project will be the potential to provide a novel transparent conducting electrode that is highly transparent in the near infrared for tandem solar cell applications. Solar energy market is currently one of the fastest growing energy sectors in the world. To compete directly with traditional fossil fuels, the cost per watt of installed solar modules must be further reduced. Thin-film based tandem solar cells have the potential to provide high power conversion efficiency and low cost. The transparent conducting electrodes that is highly transparent in the near infrared combined with highly efficient wide-band gap solar cells is expected to lead the way to low-cost, high-efficiency tandem solar cells.
Developing low-cost, solution processable transparent conducting electrodes is an important area of nanoscience research that can lead to practical solutions in the next 3-5 years in a variety of applications such as photovoltaic cells, electrochromic windows, LEDs, and flat screen displays. The key function of transparent conducting electrodes for most applications is to provide maximum transmission of light while minimizing I2R power losses due to resistive heating. For photovoltaic applications a sheet resistance <20 ohms/square with a transmission >85% is often required. For certain photovoltaic cell applications the transparent conducting electrode must survive temperatures greater than 400°C. NanoMarkets estimates that the market for solution processed TCEs made from nanomaterials will be $540M in 2016. Silver nanowire based transparent conducting electrodes are beginning to penetrate the market, but these materials cannot survive temperatures greater than 200°C and are therefore not compatible with many high temperature processing applications. The PLANT PV NSF Phase I feasibility study looked at developing noble and refractory metal based nanofibers using rapid thermal processing to improve stability and increase the throughput rate to levels required for commercial manufacturing. Noble metal and refractory metal-based nanofibers are significantly more resistant to oxidation than Cu and Al based nanomaterial systems. In this feasibility study we were able to precisely control the nanofiber diameter, density, and orientation during electrospinning, which will allow for design flexibility in future applications. In addition, electrospinning has been established in a variety of industries (e.g. textile and medical) as a low cost fabrication method. Our results showed that electrospun nanofibers are centimeters in length and require a slightly lower fiber density to reach the charge transport percolation threshold. While the refractory metal nanofibers were able to survive high temperature (>600°C), they did not achieve the high conductivity similar to Ag nanowire based systems, which limits their use in photovoltaic cells. In this study we determined that the lower conductivity was due to the formation of polycrystalline nanofibers with large inclusions of carbon between the grains. However, one key advantage of these nanofiber films is their high transmission in the infrared portion of the solar spectrum. Metal oxide films (e.g., indium tin oxide) that are commonly used as transparent conducting electrodes in commercial applications strongly absorb in the near infrared. The metallic nanofiber meshes achieved high transparency (>90%) over a wide range of spectral range (350 nm - 1300 nm) opening the door to a wide array of applications including NIR detectors, electrochromics, and touch screens.
